Circuit-arrangement for tone-control in low-frequency amplifiers



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Patented Aug. 25, 1953 CIRCUIT-ARRANGEMENT FOR TONE-CON- TEOL IN LQWUFREQUENCY AMPLIFIERS Henricus Adrianus Broos, Eindhoven, Netherlands, assigner to Hartford National Bank and Trust Company, Hartford, Conn., astrustee `Application June 21, 194.9, Serial No. 100,438 In the Netherlands June 25, 1948 3 Claims. `1

My copending U. 3S. patent application, Serial No. 790,102, led December 6, 1947, now Patent No. 2,547,739, issued April 3, 1951, describes a tone control circuit for a low frequency amplier wherein a negative feedback voltage, derived from a potentiometer and varying in frequency in accordance with the position of the `movable contact of the potentiometer, is applied to the grid circuit of at least `one of the amplifying tubes of the amplifier. In one of the circuit arrangements described in my said patent application, a negative "feedback voltage for the middle frequency range `is applied to a second fixed contact of the potentiometer and a negative feedback potential for the high frequency range is applied to a movable contactof the potentiometer, a third fixed contact of the potentiometer being coupled to the grid of .the amplifier tube.

The principal object of the present invention to provide an improved tone control circuit arrangement.

More particularly, itis an object of the invention to provide a simplified and `more efe icient tone control circuit of the type described in my said copending patent application.

Another object of the invention is to provide a tone control circuit which may be used advantageously in combination with a physiological tone control.

Further objects of the invention will appear from the following description.

The present invention is based on recognition of the fact that a low-frequency amplifier' arrangement, more particularly an arrangement used in the low-frequency amplifying stage of a wireless receiving set, frequently comprises separate negative feedbacks or other `means operative to bring about an attenuation of the intermediate tones relatively to the low and high tone frequencies. More particularly, such a negative feedback of the intermediate tone frequencies is found if a physiological control is provided in the receiver, that is to say a control in which, if the amplifier is adjusted for a high gain by use of a volume control, the low tone frequencies are amplified to a smaller extent relatively to the intermediate and high tone frequencies than if the volume control is adjusted for a low amplifier gain. In suchan ampliiier the invention permits simplification of the tonei control circuit-arrangement, and in particular the intermediate tone frequency filter.

According to the invention, the intermediate tone frequencies are attenuated inthe preceding channel of theloW-frequency amplier relatively to the low andhigh tone `frequencies. In this case it is not necessary to :provide an intermediate `tone lterbetween the source of output voltage and the potentiometer. Instead, it is merelynecessary to intercouple one of the contacts of the potentiometer and ground through a `filter adapted to pass the intermediate and high tone frequencies.

:In order that the `invention may be more clearly understood and readily carried into effect, one form will now-be described more fully by Wayof example `with `reference to the accompanying drawing, which is a schematic circuit diagram. f i

Referring now to the drawing, a low frequency amplifier is shown in the upper right hand corner thereof. This amplifier comprises an electron discharge `tube 3. `A second electron discharge tube is coupled in cascade with tube 3. To the anode circuit of *this second -tube there is coupled an output transformer 4. The grid circuit `of `tube 3 comprises a potentiometer `Shaving fixed contacts a, b and `c and a movable contact Fixedcontact a is coupled to a point A on the secondary winding of transformer 4 through a e filter including resistor 5 and a capacitor 1.

This filter is adapted to pass low frequencies and servesto apply a negative feedback potential for the low frequency range to the point a of potentiometer 5. Conductor Iserves to interconnect the fixed-contact c and the grid of tube 3. The movable contact of potentiometer 5` is coupled to the secondary winding of output transformer 4 through `a capacitor I4. Capacitor I'4, `the value of which is selected so as to pass solely` the high frequency range, serves 'to apply a negative feedback voltage for the high frequency range to the potentiometer 5. Capacitor I 4 isshown in the drawing as` being connected to point A of `output `transformer 4. However, capacitor I 4 mightbe connected tosome other point on the output transformer, such as point E. 'Ihe arrangement thus far-described provides a `large negative feedback voltage' for the high frequency range when the moving contact of the potentiometerb is located in the vicinity of xed Contact c. If the movable contact of the potentiometer 5 is located near the fixed contact a; the high frequency negative feedback voltage is sharply reduced. Adjustment of the movable contact to the fixed contact a is therefore suitable for reproduction of speech, whereas adjustment to the point c is suitable Aforthe reproduction of music. e 1

The influencing of the intermediate tone fre- Vvmits the tone setting quencies is brought about, according to the invention, in that in the `channel preceding the amplifying tube 3', comprising blocking resistance l5, a high negative feedback for the intermediate tone frequencies is effective, for example, in that a voltage which is proportional to the output voltage is Supplied, through an Y intermediate tone filter, to the grid circuit of a discharge tube. In addition, the xed contact b of potentiometer 5 is connected through a capacitor to a point of constant potential, in this case ground. Capacitor 20, constitutes a filter adapted to pass at least the intermediate and high tone frequencies. The value of capacitor 20 may be, for example, between .05 and .l F.

`Since in the preceding stage a high degree of negative feedback of the intermediate tone frequencies occurs, the oscillations supplied through the conductor I to the amplifying tube 3 will have an attenuation of the intermediate tones relatively to the low and the high tones. Since, in the grid circuit of the amplifying tube 3 a frequency-dependent negative feedback voltage appears, the frequency, relationship of which is determined by the position of the sliding contact of the potentiometer 5, the nal amplifying characteristic curve is such that, if the sliding contact is located in the proximity of the fixed contact a of the potentiometer, both therlow and the intermediate tone frequencies are negatively fed back (speech position), whereas, if vthe said sliding contact is in the Y* proximity of the fixed contact c, both the intermediate and the high tone frequencies are negatively fed back (mellow position). If the sliding contact o-f ed in the proximity of the fixed contact b, which is connected to earth through the condenser 2D, a negative feedback of the high tone frequencies will no longer take place, since the impedance of the condenser 20 is low relatively to the condenser I4, which permits passage of only high tone frequencies. At the same time -a negative feedback of the low tone frequencies is reduced to a marked extent, since the negative feedback voltage operative across the condenser 1 is divided as to voltage as far as the low tone frequencies are concerned across the impedance of the part of the potentiometer -5 which is intermediate the contacts a and b of the potentiometer and those of condenser 20. The arrangement is thus materially simplified without any loss of the advantages which perto be adjusted continuously from that for speech reproduction tomusic reproduction 4and mellow reproduction without the amplification of the intermediate-tone range, to which the human ear is most sensitive beiner modified due tothe tone control.

The circuit-arrangement according to the invention may be used with particular advantage inconjunctionwith the so-called physiological tone control described'in my copending U. S. patent application Ser. No. 17,243, filed March 26, 1948, now Patent No. 2,569,973, issued October To illustrate application of the invention to a circuit arrangement employing physiological tone control, the grid circuit of amplifying tube 3 is provided with a voltage obtained from the sliding contact 26 of the gain control 24, tothe top 31 of which the low-frequency oscillations yielded in the output `of a detector stage including an input lcircuit 2l. a diode tube 22, and

the potentiometer 5 is locatand an anode output circuit,

' quencies is an output network 23, 25 are supplied through a resistance 23 and whose lower end 3| is connected to a source of negative feedback voltage, for example, the tapping B of the output transformer 4 of the amplifying Vcircuit arrangement. With the use of resistances 34 and 43, the top 3l of the gain control 24 also has supplied to it a positive feedback voltage obtained from the tapping D of the output transformer 4, so that at this point'substantially no negative feedback is effective. Capacitor 33' and resistor 34 which intercouple point '32 of potentiometer 24 and the point D of output transformer `4 and capacitor 35 and resistor 33 which intercouple point 32 of potentiometer `24 and point B of output transformer 4 serve as intermediate frequency tone filters. These filters insure that, if the movable contact` 26 of gain control 24 is moved downwards, corresponding to :a decrease in gain of the circuit arrangement, the negative feedback` for the intermediate tone freincreased. Since an amplifier comprising the physiological tone control-comprises the circuit'elements 33, 34, 35 and 36, combination of such an arrangement with the circuit of the invention combines the advantages of the invention with a physiological tone control with a minimum of circuit elements.

While my invention has been `described in a particular use thereof and in a particular embodiment, I do not wish to be limited thereto, for obvious modifications thereof will occur to those skilled in the art without departing Vfrom the spirit and scope of my invention.

What I claim is:

l. A circuit arrangement for amplifying an incoming signal voltage having frequency components within a given range, said arrangement comprising a physiological gain control network having a frequency response characteristic'within said given range at which the intermediate frequency portion thereof is attenuated relative to the high and low-frequency portions thereof, first andV second electronic amplier stages each having an electron discharge tube provided with a cathode, a grid and an anode, and a grid input the output circuit of said first stage being coupled to the input circuit of the second stage, means to supply said signal voltage through said network to the input circuit of said rst stage whereby an output voltage is Y produced across the output circuit of said second stage, and apparatus to vary the frequency response of said arrangement, said apparatus including first, second and third frequency selective circuit means coupled to the output circuit of said second stage to derive respectively from said output voltage a low-frequency negative feedback voltage, an intermediate frequency negative feedback voltage Vand a high-frequency negative feedback voltage, means to apply said intermediate frequency voltage to said network to vary the intermediate' frequency attenuation thereof, a potentiometer provided with first, second, third and fourth contacts, said fourth contact being movablaone of said third and fourth contacts being connected tothe grid of the first stage, the rst contact being coupled to the cathode of said first stage, means to apply said lowfrequency voltageV to said first Contact, means to apply said high-frequency voltage tc the other of said third and fourth contacts and means having a low impedance value at intermediate and highto its valueV at low-frequensecond contact to a point of frequencies relative ci$ @Onnccting said ground potential whereby movement of said fourth contact along said potentiometer varies the frequency response of said circuit arrangement.

2. A circuit arrangement as set forth in claim l wherein said one of said third and fourth contacts is the third contact, and said means connecting said second contact to a point of ground potential includes a capacitance.

3. A circuit arrangement as set forth in claim 1 wherein said one of said third and fourth contacts is the fourth contact.

HENRICUS ADRIANUS BROOS.

References Cited in the iile of this patent Number UNITED STATES PATENTS 

